Winding connection for single phase two leg electric transformer

ABSTRACT

A single phase power transformer having a low power tertiary winding comprises a two-legged core with four concentric coils on each leg. The radially innermost coils are of unequal turns and series connected to constitute the tertiary winding. The high voltage main winding comprises the two radially outermost coils of like formation connected in parallel. The low voltage main winding comprises two series coil circuits in parallel, each series circuit including a radially inner intermediate coil on one leg and a radially outer intermediate coil on the other leg, the radially intermediate coils on each leg having unequal numbers of turns.

United States Patent [72] Inventor Wallace M. Johnson Pittsfield, Mas. [2i Appl. No. 860,757 [22] Filed Sept. 24, 1969 [45] Patented May 18, 1971 v [73] Assignee General Electric Company [54] WINDING CONNECTION FOR SINGLE PHASE TWO LEG ELECTRIC TRANSFORMER 5 Claims, 3 Drawing Figs.

[52] [1.8. CI 336/170, 336/183, 336/184 [51] Int. Cl 11011 27/28 [50] Field of Search 336/ 170, 171,180,l81,183,184,182

[56] References Cited UNlTED STATES PATENTS 729,748 6/1903 Frank 336/ 1 84X 873.036 12/1907 Frank 1,706,193 3/1929 Stephens ABSTRACT: A single phase power transformer having a low power tertiary winding comprises a two-legged core with four concentric coils on each leg. The radially innermost coils are of unequal turns and series connected to constitute the tertiary winding. The high voltage main winding comprises the two radially outermost coils of like formation connected in parallel. The low voltage main winding comprises two series coil circuits in parallel, each series circuit including a radially inner intermediate coil on one leg and a radially outer intermediate coil on the other leg, the radially intermediate coils on each leg having unequal numbers of turns.

WINDING CONNECTION FOR SINGLE PHASE TWO LEG ELECTRIC TRANSFORMER My invention relates to electric transformers of the singlephase type having two main high power windings and at least one or more auxiliary windings on a magnetizable core having two active legs, each winding having at' least one section thereof on each core leg. The invention is especially adapted to very high voltage transformers and particularly to high voltage autotransformers wherein insulation space becomes a major design problem.

Low voltage auxiliary windings are frequently provided on power transformers for the purpose of supplying low power loads such control equipment or for connection in delta in multiphase connections of single-phase transformers to carry phase zero sequence and third harmonic currents. Such auxiliary windings typically are of a low power rating and small current-carrying capacity so that they may be damaged inthe event of short circuit unless the auxiliary winding leakage reactance to other windings is sufficiently high to limit current to an acceptable value. Auxiliary windings being typically of low voltage are usually located immediately adjacent the core in any group of radially superposed concentric windings. It is therefore difficult to obtain high leakage reactance in auxiliary windings because of the high degree of mutual coupling with the main windings and thus the relatively small leakage flux between the main and auxiliary winding.

In the case of a two-leg single-phase transformer, it is known that the reactance of an auxiliary winding having sections on each leg connected in series circuit relation may be increased if the auxiliary winding sections (i.e., coils) have unequal numbers of turns and the two sections of one or both of the main windings areconnected in parallel. In such an arrangement the unbalanced ampere turn relation between the series connected auxiliary winding sections forces a circulating current to flow in the parallel connected main winding sections and the resulting increase in leakage flux between the main and auxiliary windings increases the reactance from other windings to the auxiliary winding.

When the auxiliary winding sections of a two-leg single phase transformer are unequal as above, it has been found desirable that the main winding having parallel connected sections be that winding which utilizes the radially outermost coils in the two superposed concentric groups of coils because the leakage flux between the innermost and outermost coils on each leg is greater than in respect to the intermediate windings. The outermost coils are typically those in the high voltage winding for well-known insulating reasons. If the low voltage main winding formed of the intermediate coils is formed by the usual parallel connection of these coils, it is found that only limited increase in auxiliary winding reactance may be attained. This is because the circulating current coupling between the unbalanced auxiliary winding sections is greater with respect to the radially adjacent intermediate coils than with respect to the outermost coils. A limit of reactance increase is therefore imposed by the relatively smaller leakage reactance between the inner and intermediate coils than between the inner and outermost coils. In a limiting case auxiliary winding reactance can only be doubled by parallel connection of the intermediate coils even if all turns of the auxiliary winding are on one leg. Such extreme unbalance however is undesirable for other reasons. On the other hand, if the low voltage main winding coils are connected in series, the resulting structure is especially disadvantageous in autotransformer connections because of the greater insulation distances required.

It is desirable, therefore, in a two-leg single phase transformer, especially a high power autotransformer having an innermost auxiliary winding, that both main windings be formed by parallel connection of coil sections while preventing the flow of circulating current in all parallel connected coil sections except the radially outermost.

In carrying out my invention in one preferred embodiment, I construct a single-phase autotransformer on a magnetizable core having two active coil-carrying legs in parallel spaced relation and four pairs of winding coils. One coil of each pair is located on each core leg, and the coils on each leg are in radially spaced concentric relation. A low voltage auxiliary winding is formed by connecting the two radially innermost coils in series circuit relation, one such coil having more turns than the other. The high voltage series winding is formed by corinecting substantially identical radially outermost coils in parallel circuit relation. The low voltage common winding comprises two series circuits each comprising an inner intermediate coil on one leg and anouter intermediate coil on the other leg, the two series circuits being connected in parallel and the series circuit cross-connections between core legs being so offset from the turn midpoint of each series circuit that no circulating current is induced in the common winding as a result of ampere-turn unbalance in the series-connected auxiliary winding coils.

Accordingly it is a general object of my invention to provide improved means for limiting short circuit current flow in a low power auxiliary winding inductively coupled to the main windings of a power transformer.

It is a more particular object to provide a high voltage autotransformer having main windings connected to maximize the reactance of an inductively associated auxiliary winding of low current capacity without requiring increase in insulation spaces.

Specifically it is an object of my invention to provide, in a single-phase high voltage transformer having two active core legs with high voltage and low voltage main windings partially on each leg, an improved winding connection permitting parallel connection of coil sections in both main windings without unfavorably affecting the reactance of an inductively associated winding of low voltage power capacity.

My invention will be more fully understood and its various objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawings in which:

FIG. 1 is a side elevational view partially in section of a power transformer having main and auxiliary windings arranged and connected in accordance with my invention;

FIG. 2 is a schematic connection diagram for the transformer windings of FIG. I showing the windings physically disposed on two separate core legs in parallel spaced-apart relation; and

FIG. 3 is a schematic circuit diagram illustrating more particularly the circuit relationship, of the windings shown at FIG. 2.

Referring now to the drawing and particularly to FIG. 1, I have illustrated a high voltage electric autotransformer comprising a magnetizable core 1 having two active coil-carrying legs A and B in parallel spaced-apart relation and parallel connecting yokes y and y bridged upon opposite ends of the core legs to form a continuous magnetic loop. On each core leg A and B there is mounted in radially superposed concentric relation a set of four coils, hereinafter referred to also as winding sections. The four coils or winding sections mounted upon the leg A, proceeding radially outward from the innermost, are identified as T L1 L2,, and H Similarly the four coils on leg B are identified as T,,, LI L2 and H The windings on the transformer shown at FIG. 1 are connected as an autotransformer having a low voltage or common winding between a grounded input terminal 10 and a high voltage input terminal 11, a series winding section between the tenninal 11 and a higher voltage terminal 12, and an electrically isolated auxiliary or tertiary winding of low voltage rating having terminals 13 and 14. Preferably the magnetizable core 1 is also connected to ground as indicated at 15. At FIG. 1 each winding section or coil is shown in block cross-sectional form, and the interconnections are shown only diagrammatically. While the internal circuit through each coil is not shown at FIG. 1 it will be evident from the external connections shown that on each leg the several coils or winding sections are arranged in similar radial superposition with the auxiliary winding sections T and T radially innermost and the sections on each leg are shown in parallel spaced relation and Y in therelatiVe radial superposition described, the core legs being schematically illustrated as in'parallel spaced relation; at FIG. 3 the same coil connection is illustrated in simplified schematic form.

Referring now more particularly to FIG.-2. it will be observed that the auxiliary winding comprises the two radially innermost coils T and T, connected in series circuit relation. In

I, a transformer embodying my invention the coils T, and T,, are .similar but not, identical in that more turns are provided on one of these coils than on the other so that a predetermined amount of unbalance exists in the ampere turns provided by these two coils. The direction of current and voltage drop through the auxiliary winding is indicated by the small arrows associated with the, winding sections. A

11 and the high voltage outputterminal 12. This windingis' comprised of the sections H, and H connected in parallel circuit relation, the low voltage end of each coil being connected to the terminal ,lland the high voltage end of each coil being connected to the terminal 12 as illustrated by the associated arrows. A

In accordance with my invention the low voltage main winding, i.e., the common winding in the case of the autotransformer illustratedQeomprises a series and parallel connection of the four radially intermediate coils or winding sections consisting of Ll,and L2, on leg A, and L1,, and L2,, on leg B. L1, and L1,, designate the radially inner intermediate section, and L2 ,and L2,, designate the radially outer intermediate sections. To constitute the low voltage main winding each radially inner intermediate coil on one leg is connected in series with'the radially outer intermediate coil on the other leg in aiding voltage relation and these two series circuits are connected in parallel between the input terminals and 11, the low voltage end of each series circuit being connected to the terminal 10 as indicated by the associated arrows.

In the transformer illustrated at FIGS. 1, 2 and 3 described above, the high voltage winding sections H, and H,, are substantially identical, having the same number of turns and the I vsame number of layers and the same diameter and being similarly disposed on the core legs A and B. These winding sections being connected in parallel circuit relation provide a loop circuit in'which a circulating current will flow to balance any ampere turn unbalance otherwise imposed upon the core legs A an B. As previously pointed out the auxiliary winding sections T, and T, are intentionally made unequal in their numbers of turns andbecause of this inequality an unbalanced ampere relation between the legs A and B does normally force a compensating circulating current to flow through the high voltage winding sections l l,and H,,. As is well known to those skilled in the art the increased leakage flux between the innermostand outermost coils or winding sections on each core leg the flow of circulating current in the manner described the leakage flux between the innermost and outermost winding sections on the core legs A and B is in effect amplified by a second transformation thereby to increase the leakage 4 reactance of the low power auxiliary winding sections T, and T It is desirable that this circulating current effect take place between the radially most remote winding sections since the leakage flux between theseisections is maximum.

In accordance with my invention the low voltage ing comprised of the coils or winding sections L1,, L2,, L1,, and L2,, is also connected in parallel so that a loop circuit is provided within which circulating current may beforced to flow unless otherwise compensated. If all four sections of the low voltage winding had equal numbers of turns, i.e., if the inner intermediate and outer intermediate coils on each leg were equal in their number of turns, the parallel loop circuit of the low voltage winding shown at FIGS. 2 and 3 would carry circulating current in the same manner and for the same reasons'as that described above in connection with the high voltage main winding. Moreover such circulating current in the low voltage winding would predominate because of the radial proximity of the low voltage winding sections to the unbalanced auxiliary winding sections. An increase in reactance caused by circulating current therefore would be controlled primarily by the low voltage main winding and would be limited in magnitude because of the relatively smaller leakage flux between the innermost winding sections and the adjacent intermediate winding sections.

In accordance with my invention, therefore, the cross connection between core legs in each of the series circuits connected in parallel to constitute the low voltage or common winding is offset from the midpoint of each series circuit by an amount sufficient to counteract or compensate the unbalanced induced voltage otherwise tending to force circulating current in the low voltage winding. By offset from the midpoint, I mean that the inner intermediate and outer intermediate coils or windingsections on each leg have unequal numbers of turns, the offset or inequality being the same in each adjacent pair of intermediate coils, i.e., coils Ll and L2 on leg A differ in their numbers of turns in the same manner and to the same degree as the coils L1,, and L2 on leg B. The effect of this is that the cross connection in the parallel low voltage winding circuits from one leg to the other, for example the cross connection from coil Ll to L2,, in one series circuit at FIG. 3 takes place at a turn location offset from the midpoint between terminals I0 and 11. The cross connection in the other series circuit comprising the windings L1,, and L2, is similarly offset. By proper selection of the amount of such unbalance, or cross connection offset, it is possible to so design the parallel-connected series circuits constituting the low voltage winding between terminals 10 and I1 that the sum of the flux linkages in each series circuit is substantially the same. Induced voltages around the loop circuit formed by the parallel connection of the two series circuits Ll L2,, and L1,, L2, are thus in balanced opposition and the net voltage drop is substantially zero, so that any ampere turn unbalance in the auxiliary winding section T, and T, will induce a compensating circulating current in the radially outermost high voltage winding sections H I-I,,. By this offset cross connection between parallel core legs, it is possible therefore to connect low voltage radially intermediate winding sections in parallel circuit relation, thereby to optimize insulating spacing with respect to parallel connected radially outermost high voltage main windings, while limiting the flow of circulating current to the high voltage winding for the purpose of increasing reactance of an innermost auxiliary winding.

At FIG. 3 l have shown an elementary diagram of connections for the autotransformer of FIGS. 1 and 2, the parallel connected high voltage and low voltage main windings being so located that the internal loop circuits are clearly shown. The cross connections between core legs in the low voltage winding form two series circuits L1,, L2,, and L1,,- L2,. In each series circuit the two included winding sections have unequal turn numbers, the inner intermediate coils L1 and L1,, in each circuit ordinarily having fewer turns than the outer intermediate coils L2 L2,.

main wind- The direction in which the series circuit cross connections are offset from the turn midpoint in each circuit is detennined by physical position of the coils. The amount of offset will be understood by those skilled in the art from the following consideratioris. i 1

It will be recalled that the series connected auxiliary winding sections T an'd T a are of unequal turn numbers, so that a short circuit of the auxiliary winding tends to unbalance the ampere turns and thus the flux linkages as between the two core legs. It is desired that compensation for this effect be provided by the-high voltage main winding, and that the sum of induced voltages around the low voltage main winding loop be zero. I

Any increase in ampere turns in the low voltage main coils on one core leg, for example the coils L1,, L2,, on leg B increases leakage reactance and thus voltages in these coils with respect to the corresponding coils on core leg A. But since L2,, is radially beyond L1,, the increase inleakage reactance is greater in L2,, than in L1,. This inequality of increase in leakage reactance may be compensated by offset of the series circuit cross connections from the series circuit midpoints, the

amount and direction of offset being so selected that, takinginto account the compensating effect of circulating current in the high voltage winding loop, the same total number of flux linkages exist'in each series circuit in the low voltage winding loop, Those skilled in the art will understand that in this relationship total flux linkage in a circuit is the sum of the flux linkages through each coil turn in the circuit.

lt will now be evident to those skilled in the art that while! have illgrstrated my invention as applied to a-transfonner of the ant transformer type,.it is equally applicable to transformers hav ing electrically isolated low voitage and high voltage main windings. Moreover, while it is not limited in its application to transformers of very high voltage ratings, it is especially applicable to such transformers for the reason that by means of this invention an optimum increase in auxiliary winding reactance may be attained without increasing the spacing between windings for this purpose and without requiring the additional insulating spacing characteristic of transformers in which one main winding of a two-leg single-phase transformer is parallel connected while the other main winding is series connected.

Thus while l have illustrated only one preferred embodiment of my invention, it will be evident to those skilled in the art that many modifications may be made, and I therefore wish to have it understood that I intend herein to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

' lclaim: r

1. In a single-phase electric transformer having a two-legged magnetizable core and a plurality of inductively coupled winding coils on each leg of said core, said winding coils on each said core leg being radially superposed in concentric relation, an auxiliary winding comprising the innermost coil on each said core leg, said innermost coils having unequal numbers of tumsand being connected in series circuit relation, a high voltage main winding comprising the outermost coil on each said core leg, said outermost coils having equal numbers of turns and being connected in parallel circuit relation, a low voltage main winding having two radially intermediate coils on each said core leg, each outer intennediate winding coil having equal numbers of turns and each inner intermediate winding coil having equal numbers of turns, and means cross-connecting the outer intermediate coil on each core leg in series with the inner intermediate coil on the other core leg and connecting said cross-connected series circuits in parallel to constitute said low voltage main winding, each adjacent pair of inner and outer intermediate winding coils having such turn relation that the sum of the flux linkages in each said series circuit is substantially the same.

A single-phase electric transformer of high transmission voltage rating comprising a magnetizable core having two active legs in substantially parallel spaced relation, and four inductively coupled winding coils on each said core leg, the coils on each said core leg being radially superposed in concentric relation, the radially innermost pair of coils on said legs having unequal numbers of turns and other similarly positioned pairs of coils having equal numbers of turns, means connecting said radially innermost pair of coils in series circuit relationto con stitute an auxiliary winding of low current and voltage rating, means connecting the radially outermost pair of coils in parallel circuit relation to constitute a high voltage main winding, and means cross-connecting the outerintermediate coil on each core leg in a series circuit with the inner intermediate coil on the other core leg and connecting said series circuits in parallel to constitute a low voltage main winding, adjacent inner and outer intennediate coils on each said core leg having a differential turn relation sufficient to prevent the flow of circulating current within said low voltage main winding.

3. An autotransformer according to claim 1 in which said high voltage and low voltage main windings are electrically connected to a common terminal.

4. A single phase electric power transformer comprising a magnetizable core having two .legs in substantially parallel spaced-relation, at least four coils on each said leg radially superposed in concentric relation, the radially innennost coils on said legs being of low current-carrying capacity relative to the other coils and being of unequal numbers of tums', means connecting said innermost coils in series circuit relation to constitute a low power auxiliary winding, means connecting the radially outermost coils on said legs in parallel circuit relation to constitute a high voltage main winding, said high voltage main winding forming a loop circuit path for circulating current induced by ampere-tum unbalance in said series connected innennost coils, leakage flux due to said circulating current increasing the inductive reactance of said auxiliary winding, means connecting an inner intermediate coil on each said leg in series circuit relation with an outer intermediate coil on the other leg, said inner and outer coils on each leg having unequal numbers of turns, and means connecting said series circuits in parallel to constitute a low voltage main winding, the turn inequality of adjacent inner and outer intermediate coils compensating unequal change in leakage reactance of such coils on the respective core legs due to ampere turn unbalance of said series-connected innermost coils and providing the same total flux linkages through each said series circuit.

5. An electric power transformer according to claim 4 wherein said low voltage and high voltage main windings have a common electrical terminal and are disposed in aiding voltage relation between their electrically remote terminals, said auxiliary winding. being electrically isolated from said main windings. 

1. In a single-phase electric transformer having a two-legged magnetizable core and a plurality of inductively coupled winding coils on each leg of said core, said winding coils on each said core leg being radially superposed in concentric relation, an auxiliary winding comprising the innermost coil on each said core leg, said innermost coils having unequal numbers of turns and being connected in series circuit relation, a high voltage main winding comprising the outermost coil on each said core leg, said outermost coils having equal numbers of turns and being connected in parallel circuit relation, a low voltage main winding having two radially intermediate coils on each said core leg, each outer intermediate winding coil having equal numbers of turns and each inner intermediate winding coil having equal numbers of turns, and means cross-connecting the outer intermediate coil on each core leg in series with the inner intermediate coil on the other core leg and connecting said cross-connected series circuits in parallel to constitute said low voltage main winding, each adjacent pair of inner and outer intermediate winding coils having such turn relation that the sum of the flux linkages in each said series circuit is substantially the same.
 2. A single-phase electric transformer of high transmission voltage rating comprising a magnetizable core having two active legs in substantially parallel spaced relation, and four inductively coupled winding coils on each said core leg, the coils on each said core leg being radially superposed in concentric relation, the radially innermost pair of coils on said legs having unequal numbers of turns and other similarly positioned pairs of coils having equal numbers of turns, means connecting said radially innermost pair of coils in series circuit relation to constitute an auxiliary winding of low current and voltage rating, means connecting the radially outermost pair of coils in parallel circuit relation to constitute a high voltage main winding, and means cross-connecting the outer intermediate coil on each core leg in a series circuit with the inner intermediate coil on the other core leg and connecting said series circuits in parallel to constitute a low voltage main winding, adjacent inner and outer intermediate coils on each said core leg having a differential turn relation sufficient to prevent the flow of circulating current within said low voltage main winding.
 3. An autotransformer according to claim 1 in which said high voltage and low voltage main windings are electrically connected to a common terminal.
 4. A single phase electric power transformer comprising a magnetizable core having two legs in substantially parallel spaced-relation, at least four coils on each said leg radially superposed in concentric relation, the radially innermost coils on said legs being of low current-carrying capacity relative to the other coils and being of unequal numbers of turns, means connecting said innermost coils in series circuit relation to constitute a low power auxiliary winding, means connecting the radially outermost coils on said legs in parallel circuit relation to constitute a high voltage main winding, said high voltage main winding forming a loop circuit path for circulating current induced by ampere-turn unbalance in said series connected innermost coils, leakage flux due to said circulating current increasing the inductive reactance of said auxiliary winding, means connecting an inner intermediate coil on each said leg in series circuit relation with an outer intermediate coil on the other leg, said inner and outer coils on each leg having unequal numbers of turns, and means connecting said series circuits in parallel to constitute a low voltage main winding, the turn inequality of adjacent inner and outer intermediate coils compensating unequal change in leakage reactance of such coils on the respective core legs due to ampere turn unbalance of said series-connected innermost coils and providing the same total flux linkages through each said series circuit.
 5. An electric power transformer according to claim 4 wherein said low voltage and high voltage main windings have a common electrical terminal and are disposed in aiding voltage relation between their electrically remote terminals, said auxiliary winding being electrically isolated from said main windings. 